fix(snapshot): auto-create dir in save(), return empty list in list_snapshot_files()

[geronimo-iia]

Summary

• save() and save_any() now call create_dir_all before writing — no more Err(NotFound) on first launch when the snapshot directory doesn't exist.
• list_snapshot_files() returns Ok(vec![]) instead of Err(NotFound) when the snapshot directory is absent.
• Two regression tests added to tests/snapshot.rs.
• Version bumped to 0.3.1. CHANGELOG updated.

Fixes #7.

Test plan

• cargo test --features snapshot list_snapshot_files_missing_dir_returns_empty passes
• cargo test --features snapshot save_creates_missing_directory passes
• cargo test --features snapshot — full suite green (111 tests)
• cargo build --features snapshot — clean compile